It is known that faults, for example cracks, in the surface of a workpiece reappear after a forming process on the surface of the formed workpiece. Furthermore, faults can arise on a previously perfect workpiece through the forming process or the heating prior to these processes. For the removal of such faults it is necessary first to recognize and mark these faults so that they can be ground out.
There are various processes for establishing and marking the faults. In one known process a magnetic induction is produced in the workpiece. The magnetic field is disturbed at the points where there are surface faults. This effect is detected either by means of sensing elements in the form of coils or magnetic tapes and, for example, by spraying on color, or is simply distinguished by magnetic powder being scattered on the whole of the surface. The distribution of magnetic powder can be differentiated on the points where there are surface faults from the distribution of the faultless areas of the surface, thus, enabling these points to be marked manually. This marking, however, is lost if an operator operates the grinding device at the point where the fault is to be found. This is disadvantageous, for the operator often cannot recognize, or at least can only recognize with great difficulty, whether the grinding process is sufficient to completely remove the fault. It is therefore necessary to re-check the workpiece after grinding and to regrind the workpiece if it still shows faults.